The oxidation of a lubricating oil often leads to the formation of oil-insoluble products which form particles that separate from the oil in a form of sludge, lacquer or varnish-like coating on engine parts.
Neutral and overbased alkaline-earth metal phenate sulfides have been known for their excellent dispersing properties and the ability to neutralize engine acids. Use has been found for these compositions primarily as additives in lubricating oils.
Overbased metal phenate sulfides contain high levels of alkali or alkaline earth metal and possess strong detergent power and an even greater capacity to neutralize acidic contaminants than the neutral metal phenate sulfides. However, overbased metal phenate sulfides which contain high levels of metal have been found to be undesirably viscous in many cases. The high viscosity may be related to the higher solids content of overbased products as well as the presence of multivalent metal salts which may crosslink with other molecules resulting in a more viscous product. Additionally, it has been found that above a certain level of alkalinity undissolved solids become an increasing problem. It would be desirable to increase the alkalinity level of these additives without increasing their viscosity.
The conventional method of making overbased phenate sulfide detergents involves mixing long chain alkyl substituted phenols, a source of sulfur and an excess of metallic base. Carbonation of the reaction mass follows to increase the amount of metal base homogeneously dispersed in the final product.